Technical Abstract:
Red rice (Oryza sativa L.) is a problematic weed in Arkansas rice production, and infestations have increased in the last three decades. We hypothesize that the morphologically diverse Arkansas red rice populations also have high genetic diversity and that this diversity emanates from genetic introgression between red rice and cultivated rice in more than a century of coexistence. We tested 137 red rice accessions from four different ecological zones in Arkansas and 36 rice cultivars that have been grown in Arkansas in the past century with 27 rice microsatellite markers distributed across 12 chromosomes. Results suggest that awnless strawhull red rice populations are genetically distant from blackhull (GD = 0.55) and brownhull (GD = 0.60) red rice types. Blackhull and brownhull types are most genetically distant (GD = 0.96) compared with strawhull. Within each red rice type, blackhull (GD = 0.76) and strawhull (GD = 0.68) are genetically less diverse than brownhull (GD = 0.80). The blackhull and strawhull diversity is affected by their zone of origin. Rice cultivars grown in Arkansas during the last 100 yr have a narrow genetic background. Twenty-five percent of the red rice accessions tested showed some genetic introgression from cultivated rice in the last century. Despite originating from a few introduced biotypes in the late 1600s, a diverse complex of weedy populations had evolved in a region devoid of other weedy and wild Oryza species.